1. Field of the Invention
The present invention relates to industrial water intake filtration systems, and more particularly to cartridge filters that are benign to marine life forms.
2. Background Information
Industrial plants, e.g. power, chemical, refineries, water treatment plants and the like, may extract water from nearby adjacent water resources (ocean, river, or lake) for any of several purposes. Those purposes include, but are not limited to, cooling and/or make-up water for cooling towers and similar systems, facility or equipment cleaning, water supply, or other similar purposes.
The filters used in the above applications must exclude debris and entrainment of marine organisms to prevent damage and/or plugging of the filters and other downstream structures and to minimize the impact on marine life, respectively. A large number of mechanical systems have been employed as filters. These range from simple screens, rotating screens to sophisticated static and regenerable filters. Of course, such filters must be corrosion resistant.
Typical of such systems, power plants employ two primary types of water cooling. The first and most prevalent in older plants is “once through cooling.” A plant employing this type of cooling inducts large quantities of cool water into a heat exchange system and then returns the water to its source at a warmer temperature. The second type of water cooling system, called a “recirculating cooling system,” recirculates water within a mostly closed heat exchange system and uses cooling towers or ponds to reject absorbed heat to the atmosphere. A recirculating system withdraws lesser quantities of cool water from an external source to replenish losses due to evaporation and discharges. The most advanced recirculating cooling systems use a hybrid or wet/dry cooling tower to reduce the visible plume of evaporated water under certain atmospheric conditions. The present invention could be used to advantage on all types of water intake systems but is most likely to find application in make up water for recirculating cooling systems.
Cooling water used in power plants using recirculating cooling systems must be replenished for two primary reasons: evaporation and increased solids content (in some cases salinity). Typically a level detector at the cooling site, usually a cooling tower structure, determines when cooling water level is low, and a salinity detector determines when salinity is too high. When the cooling water level is low a pump will draw in new water from a nearby lake, river or ocean. When the salinity is too high some cooling water may be pumped out and new water drawn in to reduce the salinity to acceptable levels.
When cooling water is drawn from lakes, rivers or seas containing marine life, care must be taken to not disrupt the marine life balance. However, floating debris must be excluded, and live fish and plants, fish eggs, and smaller life forms must be excluded and insignificantly harmed, if at all. Filters are used for these purposes.
Also, the locations of the water intake system entry ports in the water must not impede any navigable channels.
One type of filter used in such applications incorporates a Wedge wire filter. One such filter is described in U.S. Pat. No, 5,064,536 that issued in July of 1991 to Bratten. This patent describes a closely spaced wedge shaped wire that allows water to flow through the gaps but retain solids, which are removed by backwashing, or scraping.
Wedge wire screens in practical applications consist of two components: support rods and a wedge-shaped wrap wire. Support rods and wrap wire are welded using a special process followed by heat treatment, which enhances the screen integrity by maintaining the metal's corrosion resistance. The flat base of the wedge shaped wrap wire is directed toward the fluid flow. This produces a very smooth unobstructed surface, which will not impede the free flow of materials across the screen surface.
Backwash systems are available for cleaning the wedge wire screens. During backwashing, the wedge produces a vortex that cleans more efficiently than perforated plate or flat wire screens.
Wedge wire screens can be constructed in a wide variety of shapes and sizes from a variety of corrosion resistant alloys such as stainless steels and nickel alloys. Wedge wire Tee and Drum systems reduce installation costs as they are easy to assemble and do not require costly support structures. Their non-clogging slot design results in low head losses and a reduction in downtime caused by plugged screens. The smooth surface design and rigid construction significantly reduces abrasion, increasing the effective life and resulting in longer intervals between cleaning cycles.
A single wedge wire screen cylinder may be used for small capacities while one or more Tee screens may be used for larger capacities.
However, wedge wire systems usually have openings and flow rates that impact marine organisms, and maintenance, replacement and other related tasks leave room for improvement.
Another type of filter used in similar applications incorporates a fabric boom filter. One such filter is described in U.S. Pat. No. 5,102,261 issued on Apr. 7, 1992 to Gunderson, III. This patent describes a full water depth filter curtain comprised of treated polypropylene/polyester fabric suspended by flotation billets on the water surface and secured in place with anchoring systems. The boom has a double fabric layer with an Air Burst Technology cleaning system.
However, a filter curtain such as in U.S. Pat. No. 5,102,261 requires a very large surface area to obtain a low “through screen” flow rate and this requires a very large boom that can impact a navigable water way. The filter curtain booms have openings on the bed and wall of the river, lake, or sea that impact marine organisms as the boom cannot be sealed against the waterway to ensure that marine organisms cannot enter the boom. Significant water flows (river of tidal) in the waterway can distort the boom and further exacerbate openings under the boom, anchoring of the boom, and impact the navigable waterway. Replacement, maintenance and/or repair of the boom requires removal of the boom from the waterway and this would either allow ingress of marine organisms into the intake or require the water abstraction be stopped during the period of boom removal.